In place of conventional coating agents of organic resin type, coating agents of silicone resin type have recently been used for forming highly hard protective films on the surfaces of various kinds of materials represented by plastics, because the films formed therefrom have excellent properties, including (a) scratch resistance, (b) weather resistance and (c) flame resistance.
In those coating agents of silicone resin type, the condensation reaction between silanol groups, which enables the formation of siloxane linkage having excellent weather resistance, is adopted as a curing system for the purpose of forming highly hard, weather-resistant films.
However, the condensation reaction generally requires a long heating, so that those coating agents are in an uncured, tacky condition for a long time after application thereof. Thus, it becomes necessary to keep the applied coating agents in a clean space in order to eliminate a risk that foreign substances floating in the air, such as dust, are attached thereto before the curing is completed. In cases where materials for which high transparency is essential, such as optical materials, are treated with such coating agents, the lowering of yield and productivity becomes a problem.
Further, those coating agents have drawbacks of requiring not only a treatment of the substrate surface with a primer for achieving satisfactory adhesiveness but also low-temperature storage because of high reactivity of silanol groups contained therein.
On the other hand, as a means to heighten the productivity in the surface treatment of a substrate with a coating agent has been proposed a system in which an ultraviolet-responsive polyfunctional acrylic compound is incorporated in a coating agent to enable the coating agent to be rapidly cured by irradiation with ultraviolet rays. However, the skeletal structure of such a system is constituted mainly of organic elements, so that it is difficult for the system to attain hardness on the same level as a silicone type coating agent and, what is worse, the weather resistance of the cured system is insufficient. In other words, such a system has a draw back of lacking in suitability for a protective coating.
Further, the method of using a coating agent comprising a mixture of a silicone resin capable of causing polycondensation by heating and an organic material to be cured by undergoing polymerization upon irradiation with ultraviolet rays has been proposed (in Japanese Tokkai Hei 7-278328). The term "Tokkai" as used herein means an "unexamined published patent application".
According to such a method, the condensable silicone resin solution containing a ultraviolet-polymerizable organic compound is coated on a substrate, and dried. The coating thus formed is separated into two layers so that the surface layer is rich in the silicone resin. Then, the organic compound is polymerized with ultraviolet rays, and further the silicone resin is cured by heating. Therefore, the aforementioned method has an advantage in that it can save the step of applying a primer or the curing step of a primer can be shortened.
However, the foregoing method also has disadvantages in that the coating agent used therein has unsatisfactory stability upon storage in a solution state due to the presence of hydrolysis products of silane compounds, the thermal curing thereof requires a long time and the cured coating thereof has insufficient adhesiveness.
For the purpose of removing such disadvantages, instead of using the condensation reaction of silanol groups, it has been proposed to employ an UV irradiation-utilized curing system wherein the acrylic groups of a polyfunctional acrylic compound incorporated in a coating agent are utilized for the curing reaction by UV irradiation (Japanese Tokko Hei 1-55307, Japanese Tokko Hei 3-2168 and Japanese Tokko Sho 63-65115). The term "Tokko" as used herein means an "examined patent publication".
In such a system, however, rapid irradiation with UV light is used for curing a film of coating agent with consideration for productivity increase alone, so that a network of siloxane links cannot be formed to cause deficiency in hardness of the cured film.
With intention of heightening the hardness, on the other hand, if the condensation reaction of silanol groups is combined with the foregoing curing system, silane compounds are hydrolyzed in the presence of acid catalysts, similarly to the cases where organic solvents are present therein. However, the hydrolysis products obtained are unstable, or deficient in storage stability; as a result, the film formed is inferior in workability.
Thus, there has not yet been known any coating agent which not only forms in a short time a tack-free, hard film having excellent scratch resistance, weather resistance and flame resistance but also ensures a high yield, or excellent productivity and workability (storage stability in the form of solution).